Exemplary embodiments of the present invention relate to a method for fabricating a semiconductor device, and more particularly, to a method for forming junctions of vertical cells in a semiconductor device.
As for a method for forming a junction by doping a region within a semiconductor device, an ion beam implantation method is widely used. The ion beam implantation method may be also referred to as a beam line implantation method.
As semiconductor devices are becoming highly integrated, more complicated vertical cells subject to 4F2 design rule have been produced, where F is a minimum feature size. However, doping a region through the ion beam implantation method are reaching limits. Vertical cells include pillar-type active regions each having sidewalls. The pillar-type active regions are also referred to as active pillars, where three-dimensional vertical cells are formed of the active pillars.
According to an example, selectively doping pillar-type active regions through the ion beam implantation method is performed at a certain angle. This is called a tilt ion implantation process.
FIG. 1 is a cross-sectional view illustrating a conventional method for forming junctions in a semiconductor device.
Referring to FIG. 1, a substrate 11 is etched using a hard mask layer 14 as an etch barrier to form pillar-type active regions 13 that are separated from each other by a trench 12.
Subsequently, an insulation layer 15 having an opening that exposes a portion of, for example, one sidewall of each pillar-type active region 13 is formed.
Since the gap between the pillar-type active regions 13 is narrow and the pillar-type active regions 13 are formed to have a certain height, a tilt ion implantation process 16 is applied during the doping process in order to form a junction 17 by doping a portion of a sidewall exposed through an opening.
The tilt ion implantation process 16 is to use a tilt angle. Therefore, some portions of a target region may not be doped due to a shadow effect 16A during the tilt ion implantation process 16. In other words, some portions of the target region may not be doped because they may be shadowed by adjacent pillar-type active regions 13.
Also, although the tilt ion implantation process 16 is used, it is difficult to form the junction 17 in a required doping concentration level and a required doping depth because the height of the pillar-type active regions 13 is high and the gap between the pillar-type active regions 13 is narrow.